Trying to make the DIY "Cheapo" AC system work

Sorry for cross posting thi sfrom the "Design" side-- I thought maybe people would have more ideas about this here. We built an "exchange chamber" system with ideas from Rod Gervais' book and from the DIY HVAC FAQ here. Instead of using the fan to blow cold air into the studio, we went w/ Rod's idea and used the fan to suck hot air out. Part of the reason we did this is because in this area we don't need AC every day--I've been using this room with just an exhaust fan for 15 years already and only in the summer or when more people are in here than me is the AC needed. Maybe this was a mistake, because even with the AC unit (10k BTU) blasting and the new "exchange chamber" quite chilly there is NO cold air entering the main room. I tried adding a small fan to help the cool air into the wooden duct near the AC unit, but on a warm day (80s inside the room) there was no effect on the inside temp at all. So: bigger exhaust fan? Use the fan to blow cold air into the duct and let the exhaustt be passive (as in the forum FAQ)? Use TWO large cylinder type fans? Any ideas would help--we are stumped. thanks Philip Perkins

Not everyone has Sketchup installed, so some exported JPGs or whatever would be good. I suspect the problem could be that the exhaust fan isn't pumping enough CFM. I would think a more powerful fan is in order. If there is a duct involved, it may have to be enlarged and/or made smoother on the inside surface to reduct static pressure. I think a supply fan may also be in order so that there is better balance of incoming vs. outgoing. How large is the main room? A larger A/C unit may be needed. Perhaps the equipment in the room is heating it faster than the air can be cooled with the current setup. Is there a thermometer in the main room? If not, perhaps the temperature is dropping a little and humidity is rising to the same "effective temperature" level (i.e., "how it feels"). If that's the case, a dehumidifier may be in order. I'm no expert, of course... Just some initial thoughts to get the ball rolling. Hopefully Rod will see this and can chime in as well. --Keith :mrgreen:

Philper, You're using 4" dryer type flex duct as your return air from the room? If not 4" - then it's not dryer type - so exactly what is it? Exactl how is it connected to the rooftop exhaust fan? Your drawing details leave much to be desired - I really can't tell how you set this up to work - so perhaps you could explain in depth. From what I can see in your drawing I offer the following: Hard connection (14x6 hard duct with 1" insulation - thus 12x4" free air opening)) from room to exhaust for suply air and then it looks like a exchange chamber to the flex duct running directly to the exhaust fan. This is not what I indicated in my book details as an acceptable method. Why in the world are you trying to do this by exhausting 100% air? .You don't need fans on both the inlet and return - either one will work fine - but part of what makes the system work is that the chamber becomes pressurized - and that along with the slight negative in the room of interest causes air to flow. You aren't loading up the cooling chamber - so now you only have the slight exhaust - and what little the through the wall unit contributes - and I have no ideas just how well either that unit itself or the room it dumps into is sealed to allow for pressure build-up. Another difference is that I don't show the supply duct ending 2' above the floor - The bottom line here appears to be that you are chocking everything off. PLease provide as much detail on exaclty what you did - photos would help too. Rod Rod

sharward wrote:

Not everyone has Sketchup installed, so some exported JPGs or whatever would be good. I suspect the problem could be that the exhaust fan isn't pumping enough CFM. I would think a more powerful fan is in order.

Seems like it, but I think trying a supply fan might be a better bet first? I read thru the old 'diet cookie" thread and noticed that his AC was in an attic ABOVE the studio, so maybe a passive system would work there. Since mine is NEXT to the studio I need to lift the cold air up thru that duct somehow, and not add a whole lot of noise. [/quote] If there is a duct involved, it may have to be enlarged and/or made smoother on the inside surface to reduct static pressure.[/quote] I think a supply fan may also be in order so that there is better balance of incoming vs. outgoing. [/quote] How large is the main room? A larger A/C unit may be needed. Perhaps the equipment in the room is heating it faster than the air can be cooled with the current setup.[/quote] I think I might have to move the computers to the new room, sicne they are the main source of heat in here. As a one man operation I hate doing this though--it makes trouble shooting much more difficult. [/quote] Is there a thermometer in the main room? If not, perhaps the temperature is dropping a little and humidity is rising to the same "effective temperature" level (i.e., "how it feels"). If that's the case, a dehumidifier may be in order.[/quote] There is and it was definitly going UP on a warm day w/ the room closed up, not down. [/quote] I'm no expert, of course... Just some initial thoughts to get the ball rolling. Hopefully Rod will see this and can chime in as well. --Keith :mrgreen:[/quote] thanks. here's a JPG I hope. Philip Perkins

rod gervais wrote:

Philper, You're using 4" dryer type flex duct as your return air into the room? Rod

Return? The flex duct runs between the "outgoing" register of the main room and the exhaust fan--it doesn't take air back into the "exchange" room. One issue w/ all this is that in the interest of economy I'd like to avoid using the AC unit whenever I can--the simple exhaust fan has worked pretty well for me for nearly 15 yrs, and I often work with a door to the outside open. But when I do client demos etc it's nice to close the door for isolation, and the heat builds up very fast, computers, people etc. I don't have to run lights in here during the day since I have a skylight (Velux, many layers, w/ inert gas between). Anyhow, if it looks like I need more and bigger fans I'll do it--but so far adding a small fan (50 CFM) to the cold air in duct hasn't moved nearly enough cool air to be worth the added noise. I don't record in here so some system noise is ok. The new little black box on this jpeg (#6) is where I thought I'd put a 2nd squirrel cage fan, with the exhaust pointed up the 14 X 6 plywood duct. If this works (ie cools the main room) then i'd try moving it away with some ductwork, since I'm pretty sure that it will be too noisy connected directly (blowing its own noise into the room). thanks for your help Philip Perkins

you cannot move 220 cfm through a 4" flex duct - i'm surprised your fan hasn't burned out. Her's a prescriptive rule of thumb Fan CFM min F.D. Max len. Min smooth.....max len. els. 50…………………....4 inch…………25……….....……4 inch ………...70 ……… 3 50……………………5 inch …………90…….....……...5 inch …….100 ………. 3 50 …………….……..6 inch………….No Limit …..…6 inch ……No Limit … 3 80……………………4 inch…………..2 NA …..…4 inch ……20 ………... 3 80……………………5 inch…………..15............... 5 inch …...100 ……….. 3 80……………………6 inch …………90 ……………....6 inch …..No Limit….. 3 100…………………5 inch………….2 NA …………5 inch …..50…………. 3 100…………………6 inch …………45 ……………6 inch …..No Limit 3 125…………………6 inch………… 15 …………....6 inch …..No Limit 3 125…………………7 inch………… 70…………… 7 inch …..No Limit 3 PIcture the air you can move through these duct sizes - and then what you need to move the air you're trying to move. Rod

You cannot make a system work if you are exhausting 100% air without importing some from somewhere - and that has to be near to the number of cfm you are exhausting. You need to loose the exhaust to the outside world and let the AC unit do its job. Rod

rod gervais wrote:

You cannot make a system work if you are exhausting 100% air without importing some from somewhere - and that has to be near to the number of cfm you are exhausting. You need to loose the exhaust to the outside world and let the AC unit do its job. Rod

The fan is exhausting the air from the main room directly out the roof--it is suspended from the ceiling and its outlet connects directly to the roof-jack and exhausts air to the outside. The main room is not airtight--air leaks in around the door and comes in thru a small vent in the skylight (which I can close. Should I have brought the warm air from the main room back into the exchange chamber? In order for your system to work does the whole room need to be airtight (to create suction to draw the cold air in)? I used to have the exhaust fan connected to the bottom of the wooden duct, sitting in a wooden box outside of the room and pulling air out of the main room (before the exchange chamber was built). thanks for your help--here's a crude side view of the inside wall of the exchange chamber, showing the positions of everything. #6 in the top view shows where I thought I might put a blower fan to help the cold air into the wooden duct. Philip Perkins

Will get some pix if that will help. Thanks for your replies. I mentioned the blower to blow cold air into the duct to the main room because that is how the system is configured in the sticky on this forum. Philip Perkins

Here are some pix--the room w/ the wooden duct, AC unit, fan and fan duct, and the other end of the warm air duct where it enters from the main room. thanks for your thoughts Philip Perkins

OK - so what you are saying is that the system you use now works sometimes - but not when it comes down to critical mass in the room with the doors shut - right? In other words it works by sucking air from other rooms into that room when it's only warm - but that this doesn't get the job done when you need AC. If that is the case then what you have to do is add a 2nd fan - and connect it to a new return air register in the room. This register has to dump the air back into the cool air room - not the outside. Just like I show in the book. Your problem is this - if you do not have a way to get return air into your cooling chamber - you will suck air out to the point where the negative pressure in the room is equal to the fans capacity - at which point you will then have just stagnant air........... the fan will run - but it won't move air. That is the reason that ducted air conditioning system have return air ducts - basically - what goes in - has to come back - in huge commercial systems sometimes - we might do 100% fresh air to a space - but then that means we need 100% exhaust to balance it out........ But it is impossible to do 100% exhaust and thingk that you could ever draw all of that air through cracks under doors, etc......... and even if you could - that doesn't create enough negative pressure in the room to steal it from another room with NO free air in it. But if you could - those babies (around the door and the small cracks in the room) would whislte something fierce.......... So, if you add the 2nd set-up like I said to - then when you need the AC - you turn off the exhaust fan and turn on the air conditioning fan and your problem will be solved. Good luck......... oh - btr - that is not 4" flex duct - it looks more like 8"......... Sincerely, Rod

rod gervais wrote:

OK - so what you are saying is that the system you use now works sometimes - but not when it comes down to critical mass in the room with the doors shut - right? In other words it works by sucking air from other rooms into that room when it's only warm - but that this doesn't get the job done when you need AC. If that is the case then what you have to do is add a 2nd fan - and connect it to a new return air register in the room. This register has to dump the air back into the cool air room - not the outside. Just like I show in the book. Your problem is this - if you do not have a way to get return air into your cooling chamber - you will suck air out to the point where the negative pressure in the room is equal to the fans capacity - at which point you will then have just stagnant air........... the fan will run - but it won't move air. That is the reason that ducted air conditioning system have return air ducts - basically - what goes in - has to come back - in huge commercial systems sometimes - we might do 100% fresh air to a space - but then that means we need 100% exhaust to balance it out........ But it is impossible to do 100% exhaust and thingk that you could ever draw all of that air through cracks under doors, etc......... and even if you could - that doesn't create enough negative pressure in the room to steal it from another room with NO free air in it. But if you could - those babies (around the door and the small cracks in the room) would whislte something fierce.......... So, if you add the 2nd set-up like I said to - then when you need the AC - you turn off the exhaust fan and turn on the air conditioning fan and your problem will be solved. Good luck......... oh - btr - that is not 4" flex duct - it looks more like 8"......... Sincerely, Rod

Thanks most extremely for your attention to my problem. The room is actually a free-standing structure, so air from the door etc is actually coming from outside. I think what you are telling me is that my current exhaust fan is ok for just sucking out warm air when that's all that I need, but that to make the exchange chamber AC work I need to shut that fan off and have a fan that pulls the air from the studio into the exchange room where the AC unit is, not exhaust it to the outside (since the AC unit is doing that itself I guess). Before I buy another fan and cut a 3rd hole in the wall I want to see if we can come up with a homemade "valve" that could go on the fan I have and direct its ouput to the roof jack (outside) or to the interior of the exchange room. Is this a bad idea? thanks very much for your thoughts Philip Perkins

The only way to do that would be with multiple motorized dampers - plus some low voltage wiring / controls to make it all work You could use one that was normally open - and another that was normally closed -so if you killed power to them they would go into that state - and then when you provided power they would switch. take a look........... http://www.homecontrols.com/cgi-bin/mai ... /sesent/00 So that plus adding a Tee after the fan........ Rod

rod gervais wrote:

The only way to do that would be with multiple motorized dampers - plus some low voltage wiring / controls to make it all work You could use one that was normally open - and another that was normally closed -so if you killed power to them they would go into that state - and then when you provided power they would switch. take a look........... http://www.homecontrols.com/cgi-bin/mai ... /sesent/00 So that plus adding a Tee after the fan........ Rod

Ah...ok, I see--the real motorized dampers plus the low voltage wiring would be expensive and probably something I'd need help with. Maybe I can come up with a manual system using a T in the exhaust duct I already have that is served by a 2nd fan in the exchange room--that dumps into there......and use the 2nd fan only w/ the AC unit. Will try and advise. Thanks very much. Philip Perkins

A damper will be required in both lines with this set up - If either fan runs they will pull air from and through the other fan......... the other fan will also make it impossible create a positive pressure situation within the cold room..... Rod

I ran a test last night. I had a 50 CFM bathroom-type fan around, so I disconnected the duct from the exhaust fan and hooked up the smaller fan so that air that it pulled out of the main room (10 x 12) would dump into the exchange chamber. I closed everything up (skylight vent and door), turned on the computers, TV monitor (CRT), 2 over head lights (2 x 75 watts) and powered speakers, and fired up the AC unit and the fan. Over 2 hrs the main room temp went from 72.5 to 76.6 F. I then turned off the fan and AC, and the room went to 79.5 F over 2 hrs. I then turned the fan and AC unit back on and let it run for 2 hrs, during which time the room temp stayed at 79.5 F. You may recall that this room is really a freestanding outbuilding; the exterior temp last night was around 55 F. What to conclude? A bigger fan? A problem with how the cold air duct is made? Was the AC unit not cycling because the exterior air was too cold (it seemed chilly in the exchange room)? Should the air being sucked out of the main room be dumped right onto the AC unit itself so it will react to that temperature, not the temp of the exchange room? (The AC unit's temp control was turned all the way down-60 F- and its fan was on high.) thanks Philip Perkins

philper wrote:

I ran a test last night. I had a 50 CFM bathroom-type fan around, so I disconnected the duct from the exhaust fan and hooked up the smaller fan so that air that it pulled out of the main room (10 x 12) would dump into the exchange chamber. I closed everything up (skylight vent and door), turned on the computers, TV monitor (CRT), 2 over head lights (2 x 75 watts) and powered speakers, and fired up the AC unit and the fan. Over 2 hrs the main room temp went from 72.5 to 76.6 F. I then turned off the fan and AC, and the room went to 79.5 F over 2 hrs. I then turned the fan and AC unit back on and let it run for 2 hrs, during which time the room temp stayed at 79.5 F. You may recall that this room is really a freestanding outbuilding; the exterior temp last night was around 55 F. What to conclude? A bigger fan? A problem with how the cold air duct is made? Was the AC unit not cycling because the exterior air was too cold (it seemed chilly in the exchange room)? Should the air being sucked out of the main room be dumped right onto the AC unit itself so it will react to that temperature, not the temp of the exchange room? (The AC unit's temp control was turned all the way down-60 F- and its fan was on high.)

Phillip, A 50 cfm fan bath exhaust couldn't move that air if someone had a gun to it's head........ I don't know what your total load is (you can always downlod the calculator at my website if you want to figure it out) but i would guess you need somewhere around 150 to 200 cfm to get there - and that fan is going to want to have a pretty decentmotor in it to over come the static pressure the system will have on it with the ducting the way it is. Rod

rod gervais wrote:

philper wrote:

I ran a test last night. I had a 50 CFM bathroom-type fan around, so I disconnected the duct from the exhaust fan and hooked up the smaller fan so that air that it pulled out of the main room (10 x 12) would dump into the exchange chamber. I closed everything up (skylight vent and door), turned on the computers, TV monitor (CRT), 2 over head lights (2 x 75 watts) and powered speakers, and fired up the AC unit and the fan. Over 2 hrs the main room temp went from 72.5 to 76.6 F. I then turned off the fan and AC, and the room went to 79.5 F over 2 hrs. I then turned the fan and AC unit back on and let it run for 2 hrs, during which time the room temp stayed at 79.5 F. You may recall that this room is really a freestanding outbuilding; the exterior temp last night was around 55 F. What to conclude? A bigger fan? A problem with how the cold air duct is made? Was the AC unit not cycling because the exterior air was too cold (it seemed chilly in the exchange room)? Should the air being sucked out of the main room be dumped right onto the AC unit itself so it will react to that temperature, not the temp of the exchange room? (The AC unit's temp control was turned all the way down-60 F- and its fan was on high.)

Phillip, A 50 cfm fan bath exhaust couldn't move that air if someone had a gun to it's head........ I don't know what your total load is (you can always downlod the calculator at my website if you want to figure it out) but i would guess you need somewhere around 150 to 200 cfm to get there - and that fan is going to want to have a pretty decentmotor in it to over come the static pressure the system will have on it with the ducting the way it is. Rod

Thanks Rod--that's what I needed to know. I'll get a 200+ CFM fan (from a place w/ a good return policy...) and try that. Will advise. THANKS Philip Perkins

I am running a test tonight w/ my "exchange chamber" setup in light of Rod's recent advice. In the pic you can see a 400 CFM "Low Sone" fan (Broan 400L) that is hooked up to the 8" duct that draws air out of my little studio room. The theory is that this fan will create enough negative pressure in the room to draw the cold air from the airconditioner into the room via a 2nd duct that opens near the floor. Tonight is cool again outside (55 ish). When I started the test everything had been off for awhile and the room was at 73.6 F. I turned the fan and AC unit on--with the AC cranked. There is a very light, cool breeze coming out of the cold air duct, and a piece of paper will stick to the outgoing duct.Computers, powered speakers, 3 75 watt inc. lights and 1 small fluorescent were turned on, and I left the room. 45 min later when I returned the temp was still 73.6. 15 min later, with me in the room (an hour since start of test): 74.1 F. 1hr 15 min: 74.3F. I turn off the two 75 watt overhead lights. Temp drops to 73.9 over 30 min. So I see that there is an effect by using this AC method, but the question is whether it is worth it and if it will have any effect when I need it most--on a warm afternoon. ( I'm definitely using a lot more electricity than I was with both a 10k BTU AC unit and a 400 CFM fan.) Should I consider actually blowing the cold air into the room (w/ another fan)? I'd get more poop out of the fan if it had less duct on it, but there is a noise factor with it despite being "low-sone". The irony is that on a night like tonight I might actually get the studio cooler by opening the little skylight vent and cracking the door a bit and just using the fan. What should I try next? thanks Philip Perkins

There is a very light, cool breeze coming out of the cold air duct, and a piece of paper will stick to the outgoing duct

Your ductwork is choking down the air flow - because at 400 cfm that return air duct should be trying to drag you in arm first....... Ands you aren't paying attention to one important thing I keep saying to you - which is:

and that fan is going to want to have a pretty decentmotor in it to over come the static pressure the system will have on it with the ducting the way it is.

Did you actually look at the curve chart for that fan? At an inch of static pressure in the vertical position it only moves 8cfm.......... The problem here is not the design - it's the application. I would also caulk the room to make it airtight......... that is something that HAS TO BE in order for this system to work. You are pushing a lot of air right outside at the AC unit - Rod

rod gervais wrote:

There is a very light, cool breeze coming out of the cold air duct, and a piece of paper will stick to the outgoing duct

Your ductwork is choking down the air flow - because at 400 cfm that return air duct should be trying to drag you in arm first....... Ands you aren't paying attention to one important thing I keep saying to you - which is:

and that fan is going to want to have a pretty decentmotor in it to over come the static pressure the system will have on it with the ducting the way it is.

Did you actually look at the curve chart for that fan? At an inch of static pressure in the vertical position it only moves 8cfm.......... The problem here is not the design - it's the application. I would also caulk the room to make it airtight......... that is something that HAS TO BE in order for this system to work. You are pushing a lot of air right outside at the AC unit - Rod

Thanks for your reply and thoughts. Sorry I have more questions as I don't understand your last post, or what I should do. What about which duct is choking the airflow? The cold air duct? The duct going to the fan? I did look at lots of charts but did not understand them. What is a pretty decent motor? Can you recommend a brand or model or type of fan? I went with the 400 Broan because it is quiet, available and a type of fan that I've used with success in two other installations. If it is the wrong thing, I would love a suggestion about what the right thing is. You say I am pushing a lot of air right outside the AC unit. Since you made that remark I'm guessing that that is a bad thing? The pic in your book seems like it has the warm air coming in near the AC unit, but it wouldn't be hard for me to move the fan farther away. Would this be better? Here's what I think I've learned from you so far: --The room to be cooled wants to be airtight. --I need a different type of fan with a different sort of motor than what I showed in my pix (suggestion?) --Different ductwork should be used (which flow? different how?) --The warm air return does not want to be close to the AC unit. There is no criticism of your design in my posts--it seems to me that it ought to work, and I've spent a lot of time and money trying to make it work so far but I'm not there yet, mostly due to my own ignorance. If anyone out there has built Rod's "exchange chamber" design and has any suggestions or even just a narrative of how you built it I would be very grateful to hear about it. My test proved to me that the concept of the system will work--it did do some cooling even as poorly executed as it was. Philip Perkins

philper wrote:

You say I am pushing a lot of air right outside the AC unit. Since you made that remark I'm guessing that that is a bad thing? The pic in your book seems like it has the warm air coming in near the AC unit, but it wouldn't be hard for me to move the fan farther away. Would this be better?

Nope - was referring to the opening alongside the AC unit - you are pushing a lot of air right outside there........... you need to caulk that.

--The room to be cooled wants to be airtight.

110%

--I need a different type of fan with a different sort of motor than what I showed in my pix (suggestion?)

This stuff gets expensive - but 1st let's look at maybe using what you have instead of typing to figure out what to buy that can overcome it......... I'm not there so I cannot slip a probe in to check the static on the duct - nor can I test the air flow at the return air duct at the grille........ so best would probably be to open things up.

--Different ductwork should be used (which flow? different how?)

Let me look at that in the morning.........

If anyone out there has built Rod's "exchange chamber" design and has any suggestions or even just a narrative of how you built it I would be very grateful to hear about it. My test proved to me that the concept of the system will work--it did do some cooling even as poorly executed as it was.

For the record this isn't my exchange chamber......... I believe the original question as to whether it would work or not came from Steve the Knightfly himself........ although I might be mistaken on that. However - the system works if properly designed and built. Air Tight is one of the more critical requirements here. Some people think that the fan should be installed on the supply duct instead of the return - but - that is exactly the same condition except in reverse - now you pull from the cold room direct - but have to pressurize the main room to create the flow to the exchange chamber - so what's the difference? I put it on the return because that pulls the motor noise away from the quiet room........ Will get back to you. Rod

I see that you started a thread in the design forum about ductless mini-split systems -- asking specifically about noise levels they generate. Are you considering bailing on this DIY cheap-o thing? In any event, be aware that the vast majority of ductless mini-split systems do not supply any fresh air, so you'll need a mechanical ducted ventilation system in order to meet strict building codes on the subject. --Keith :mrgreen:

Can you please replace that HUGE pic with one that meets forum requirements? Thanks... Steve

Please go ahead and delete the big picture--I don't have permission or access to do that on my own, it was (I guess) a stupid and pointless test and so is of no importance. I started the ductless split thread because I had seen them in many people's studio pix, and remembered some quietish ones from my travels overseas. I asked the questions because I don't know anyone locally who has one and haven't found a store that has them on hand. I have my doubts about whether we'll ever be able to make the DIY design work well enough to justify the level of electricity use it will require, but we're going to try a few more ideas before we bail. The mini split will probably turn out to be too noisy for me, but I did notice that there seemed to be systems available now for a lot less money compared to a few years ago. Philip Perkins